Molecular Evolution of the glutathione S-transferase family in the Bemisia tabaci species complex. Bemisia tabaci

During adaptation of herbivorous insects to their geographic and host-plant ranges, natural selection forces act on gene families involved in the process. The glutathione S-transferase (GST) family is considered one of the key targets. The family codes for enzymes that neutralize reactive oxygen species and phytotoxins through the conjugation of reduced glutathione. We studied the molecular evolution of the GST family in Bemisia tabaci, a complex of 35 sibling species, differing in their geographic and host ranges. We tested if some enzymes evolved different functionality, by comparing their sequences in six species, representing five of the six major genetic clades. Comparisons of the non-synonymous to synonymous substitution ratios, detected positive selection events in eleven codons of five cytosolic GSTs. Ten of them are located in the periphery of the GST dimer, suggesting a putative involvement in interactions with other proteins. Modeling the tertiary structure of orthologous enzymes, identified additional nineteen mutations in nine GSTs, likely affecting the enzymes’ functionality. Most of the mutation events were found in the environmentally-responsive classes Delta and Sigma, indicating a slightly different delta/sigma tool box in each species. At a broader genomic perspective, our analyses indicated a significant expansion of the Delta GST class in B. tabaci and association between the diet breadth of hemipteran species and their total number of GST genes. We raise the possibility that at least some of the identified changes improve the fitness of the B. tabaci species carrying them, leading to their better adaptation to specific environments.